A laser frequency comb that enables radial velocity measurements with a precision of 1 cm s(-1)

Searches for extrasolar planets using the periodic Doppler shift of stellar spectral lines have recently achieved a precision of 60 cm s(-1) (ref. 1), which is sufficient to find a 5-Earth-mass planet in a Mercury-like orbit around a Sun-like star. To find a 1-Earth-mass planet in an Earth-like orbit, a precision of approximately 5 cm s(-1) is necessary. The combination of a laser frequency comb with a Fabry-Pérot filtering cavity has been suggested as a promising approach to achieve such Doppler shift resolution via improved spectrograph wavelength calibration, with recent encouraging results. Here we report the fabrication of such a filtered laser comb with up to 40-GHz (approximately 1-A) line spacing, generated from a 1-GHz repetition-rate source, without compromising long-term stability, reproducibility or spectral resolution. This wide-line-spacing comb, or 'astro-comb', is well matched to the resolving power of high-resolution astrophysical spectrographs. The astro-comb should allow a precision as high as 1 cm s(-1) in astronomical radial velocity measurements.     

Optical frequency comb generation from a monolithic microresonator

Optical frequency combs provide equidistant frequency markers in the infrared, visible and ultraviolet, and can be used to link an unknown optical frequency to a radio or microwave frequency reference. Since their inception, frequency combs have triggered substantial advances in optical frequency metrology and precision measurements and in applications such as broadband laser-based gas sensing and molecular fingerprinting. Early work generated frequency combs by intra-cavity phase modulation; subsequently, frequency combs have been generated using the comb-like mode structure of mode-locked lasers, whose repetition rate and carrier envelope phase can be stabilized. Here we report a substantially different approach to comb generation, in which equally spaced frequency markers are produced by the interaction between a continuous-wave pump laser of a known frequency with the modes of a monolithic ultra-high-Q microresonator via the Kerr nonlinearity. The intrinsically broadband nature of parametric gain makes it possible to generate discrete comb modes over a 500-nm-wide span (approximately 70 THz) around 1,550 nm without relying on any external spectral broadening. Optical-heterodyne-based measurements reveal that cascaded parametric interactions give rise to an optical frequency comb, overcoming passive cavity dispersion. The uniformity of the mode spacing has been verified to within a relative experimental precision of 7.3 x 10(-18). In contrast to femtosecond mode-locked lasers, this work represents a step towards a monolithic optical frequency comb generator, allowing considerable reduction in size, complexity and power consumption. Moreover, the approach can operate at previously unattainable repetition rates, exceeding 100 GHz, which are useful in applications where access to individual comb modes is required, such as optical waveform synthesis, high capacity telecommunications or astrophysical spectrometer calibration.     

A frequency comb in the extreme ultraviolet

Since 1998, the interaction of precision spectroscopy and ultrafast laser science has led to several notable accomplishments. Femtosecond laser optical frequency 'combs' (evenly spaced spectral lines) have revolutionized the measurement of optical frequencies and enabled optical atomic clocks. The same comb techniques have been used to control the waveform of ultrafast laser pulses, which permitted the generation of single attosecond pulses, and have been used in a recently demonstrated 'oscilloscope' for light waves. Here we demonstrate intra-cavity high harmonic generation in the extreme ultraviolet, which promises to lead to another joint frontier of precision spectroscopy and ultrafast science. We have generated coherent extreme ultraviolet radiation at a repetition frequency of more than 100 MHz, a 1,000-fold improvement over previous experiments. At such a repetition rate, the mode spacing of the frequency comb, which is expected to survive the high harmonic generation process, is large enough for high resolution spectroscopy. Additionally, there may be many other applications of such a quasi-continuous compact and coherent extreme ultraviolet source, including extreme ultraviolet holography, microscopy, nanolithography and X-ray atomic clocks.     

Direct frequency comb spectroscopy in the extreme ultraviolet

The development of the optical frequency comb (a spectrum consisting of a series of evenly spaced lines) has revolutionized metrology and precision spectroscopy owing to its ability to provide a precise and direct link between microwave and optical frequencies. A further advance in frequency comb technology is the generation of frequency combs in the extreme-ultraviolet spectral range by means of high-harmonic generation in a femtosecond enhancement cavity. Until now, combs produced by this method have lacked sufficient power for applications, a drawback that has also hampered efforts to observe phase coherence of the high-repetition-rate pulse train produced by high-harmonic generation, which is an extremely nonlinear process. Here we report the generation of extreme-ultraviolet frequency combs, reaching wavelengths of 40?nanometres, by coupling a high-power near-infrared frequency comb to a robust femtosecond enhancement cavity. These combs are powerful enough for us to observe single-photon spectroscopy signals for both an argon transition at 82?nanometres and a neon transition at 63?nanometres, thus confirming the combs' coherence in the extreme ultraviolet. The absolute frequency of the argon transition has been determined by direct frequency comb spectroscopy. The resolved ten-megahertz linewidth of the transition, which is limited by the temperature of the argon atoms, is unprecedented in this spectral region and places a stringent upper limit on the linewidth of individual comb teeth. Owing to the lack of continuous-wave lasers, extreme-ultraviolet frequency combs are at present the only promising route to extending ultrahigh-precision spectroscopy to the spectral region below 100?nanometres. At such wavelengths there is a wide range of applications, including the spectroscopy of electronic transitions in molecules, experimental tests of bound-state and many-body quantum electrodynamics in singly ionized helium and neutral helium, the development of next-generation 'nuclear' clocks and searches for variation of fundamental constants using the enhanced sensitivity of highly charged ions.     

Monitor and control of neuronal activities with femtosecond pulse laser

Combined with the fluorescence labeling technique, two-photon microscopy excited with femtosecond pulse laser has become an important tool for neuroscience research. In this research, the calcium signals from neurons in rat cortex slice were monitored by a custom-built two-photon microscopy, and the spontaneous calcium signals and the pharmacological responses as well as the responses to femtosecond pulse laser stimulation were recorded. The results showed that the amplitude of the cal- cium signals increased in direct proportion to the corresponding electrical activities. Glutamate induced a calcium transient, but continuous application resulted in smaller response. Simultaneous monitoring of neuronal populations distinguished the neurons of different microcircuits. The femtosecond pulse laser induced local or global calcium signals in the pyramidal neurons. The approach of interrogation and control of neural activities using femtosecond pulse laser is non-contact, nondestructive, repeatable, and without any additional substrates, which will contribute to the development of neuroscience.     

飞秒激光和大功率激光的发展与应用

综述了激光技术的新发展和研究的热点,重点论述了飞秒激光技术的最新发展和它在生命科学、基因工程、信息科学、超微细加工及物理学等领域中的应用以及大功率激光技术的发展和它在国防军事领域中的应用。     

飞秒激光脉冲经小孔衍射后的近场光强节点

应用数值求解格林函数的方法,计算模拟了飞秒脉冲通过亚波长小孔的衍射光场. 首先将具有高斯时间轮廓的飞秒脉冲展开成一系列简谐光波的叠加,对每一个频率的简谐光波进行数值计算,得到该频率的衍射光场的空间分布, 然后对所有频率的衍射光场进行反傅里叶变换得到光场随时间的变化.分析发现,飞秒脉冲通过小孔后沿界面向小孔两边传播,并且在该传播过程中, 衍射光场会在介质的分界面上和在接近界面的区域中形成光强的节点.     

使用飞秒脉冲在熔融石英中进行的三维光数据体存储

通过啁啾脉冲放大技术,产生出脉冲宽度为200fs,波长为800nm的超短脉冲激光束,将这种激光脉冲紧聚焦到融熔石英体中,在聚焦点附近通过雪崩电离和多光子电离产生高密度的等离子体,并使等离子体吸收大量的激光能量,在局部产生微爆炸,形成微小的空腔,从而在融熔石英体中记录下三维数据.实验记录了20层存储数据,使存储密度达到3.75×1010bit/cm3.     

飞秒激光与硅样品作用生成纳米光栅结构

我们用脉冲宽度120 fs、波长800nm、能量密度从0.1 J/cm2~0.5 J/cm2的激光束照射硅样品和锗硅合金样品表面能够生成各种低维形貌结构。特别是将飞秒激光束散焦至直径为100μm的束斑,并以每秒1000个脉冲照射硅样品表面两秒钟时(能量密度在熔融阈值0.2 J/cm2附近),能生成周期间隔为400nm的浮雕光栅状的一维微结构。我们用飞秒激光与其诱导的等离子体波的相干模型解释了光栅状微结构的形成机理。还发现这种结构有很强的PL发光,PL峰的中心约在719nm处。该飞秒激光加工技术既简捷又稳定,在光学和微电子加工领域应有很好的应用前景。     

Dissociation of methanol in intense femtosecond laser field

Methanol was irradiated by 80 fs laser pulse, intensity range of 10¹³–10¹⁴ W/cm². A TOF-mass spectrometer was coupled to the laser system and used to detect the ions produced. The parent ions CH₃OH⁺ appeared firstly at the laser intensity of 1.4×10¹³ W/cm². While the laser intensity was gradually increased, the parent ions were dissociated and the primary ions CH₂OH⁺ were given as verified from the irradiation of deuterated methanol (CH₃OD) showing the C—H bond cracking firstly. While the laser intensity was further increased to 2.0 × 10¹³ W/cm², the C—O bonds of the parent ions also broke to give CH₃ ⁺. When the laser intensity was higher, smaller fragment ions like CH⁺, C⁺, OH⁺ and O⁺ also appeared. Among the fragment ions, only H⁺ ion yield had anisotropic angular distribution dependence on the laser polarization vector in the dissociation of methanol. All the experimental observations show that the dissociation of methanol proceeds through stepwise mechanism but not Coulomb explosion.     

Preparation of porous microstructures on NiTi alloy surface with femtosecond laser pulses

Porous microstructures on Nickel-Titanium （NiTi） alloy surfaces were prepared by linearly polarized femtosecond lasers with moving focal point at a certain speed. It was found that various novel micro- structures from feather-like ripples to cluster-like porous textures could be formed with increasing laser energy. Particularly, when the laser energy was 400 llJ, a periodic porous metal surface was gen- erated. Measurement of X-ray diffraction showed that the grains on the sample surface were refined through femtosecond laser ablation processes, but the crystal structures still kept their original states. Analysis by X-ray photoelectron spectroscopy revealed that Ni/l＂i on the sample surface was changed with an evident oxidization of titanium element under different laser energies. This investigation pro- vides a new approach to improve the biocompatibility of NiTi-based implant devices.     

中频差Zeeman双折射双频激光干涉系统的实验研究

Zeeman双折射双频激光器能够输出3~40 MHz频差的双频激光,可用作高测速双频激光干涉仪的光源。针对这种激光器的特点,以频差约7 MHz的Zeeman双折射双频激光器为干涉仪的光源,建立了中频差双频激光干涉仪测量系统。经过与商用HP5528A双频激光干涉仪的比对测量实验得出,中频差双频激光干涉仪实验测量系统测量线性度小于0.002%,测量的标准不确定度小于0.1μm,证明了中频差双频激光干涉系统的可行性和针对这种光源采用的设计方案的正确性。     

基于独立分量分析的跳频通信抗梳状阻塞干扰方法

针对梳状阻塞干扰对跳频通信造成严重影响的问题,根据跳频信号与梳状干扰的统计独立特性,将独立分量分析引入跳频通信抗干扰之中,提出了基于独立分量分析的跳频通信抗梳状干扰方法。围绕输出信号的统计独立性构建对照函数,利用对照函数引导分离矩阵迭代,进而将分离矩阵作用于接收混合信号,实现跳频信号与干扰信号的有效分离。仿真结果表明：提出的方法可有效提高跳频通信对抗梳状阻塞干扰能力;当误码率为10-3时,提出的方法可使跳频通信的抗干扰能力提升约8 dB。     

多方法缓解飞秒激光联合准分子激光原位角膜磨镶术后早期疼痛的效果

目的:观察不同方法缓解飞秒激光联合准分子激光原位角膜磨镶术(FS-LASIK)后早期疼痛的效果.方法:2019年1月至2020年10月在首都医科大学宣武医院随机选取双眼行FS-LASIK术的患者110例共220只眼,其中左眼为治疗眼,右眼为对照眼.将所有患者分为4组,对于治疗眼分别应用冰水、普拉洛芬滴眼液、角膜绷带镜以...     

Photodissociation/photoionization processes of chlorobromomethane induced by femtosecond laser pulses with pump-probe scheme

The photodissociation/photoionization processes of chlorobromomethane （CH2BrCl） induced by femtosecond laser pulses have been investigated using pump-probe scheme combined with the time-of-flight mass spectra. The dominate photoproducts are observed at different delay time of the pump （400 nm） and probe （800 nm） pulses and the corresponding time-dependence of them is obtained. The results show that the decaying time of the molecule CH2BrCI in the A-band is in the 100 fs. The decaying tendencies of the fragment ions （CH2CI^＋ and CH2Br^＋） and the parent ion （CH2BrCI^＋） are almost the same and the relative ratios of the yields of them keep constant during the delay time of 0 to 150 fs. These facts suggest that the fragment ions come from the fragmentation of the parent ions in excited electronic states. The probabilities to form CH2CI^＋ and CH2Br^＋ are obtained from the relative ratio of the ion intensity and are about 71.6% and 14.2%, respectively.     

用共振法测定弹性梁固有频率及振型

利用共振法测定两端自由弹性梁的固有频率和振型．将实验结果与理论值进行比较，并对共振法和测试结果进行了探讨．     

不同光斑模式的CO2激光在镀锌板钎焊连接中的应用

为了获得CO2激光填丝钎焊镀锌钢板的工艺特性,对CO2激光进行了光束成形,分别研究了圆形光斑、双焦点光斑和矩形光斑3种光斑模式下的镀锌板激光填丝钎焊工艺特点与接头质量．结果表明,采用双焦点的CO2激光光斑加热,钎焊过程适应性增加,并可获得良好的焊缝成形,同时提高激光能量利用率;矩形光斑钎焊可降低镀锌层烧损,但焊接适应性和激光能量利用率降低．通过改变CO2激光的光束能量分布,可以替代Nd：YAG激光进行镀锌板的连接．     

Ionization and dissociation of acetonitrile by intense femtosecond laser pulse

Photoionization and photodissociation of CH3CN were studied by a linear time of flight mass spectrometer coupled with 800 nm, 50 fs laser pulses at intensities of 6.3×1013-1.2×1014 W/cm2. The laser power dependences for principal ions CH3CN+, CH2CN+, CHCN+ and CCN+ were measured, which are consistent with the numbers of photons required to produce the ions via multiphoton ionization and dissociation. The results show that eight-photon non-resonant multiphoton ionization is the main photoionization mechanism of the parent ion CH3CN+, while the fragment ions were produced through the dissociation of the molecules in the super-excited states.     

PZT压电厚膜的飞秒激光烧蚀及图形化研究

利用飞秒激光实现了对PZT厚膜(十几至上百微米)材料的精确烧蚀和图形化研究.飞秒激光脉宽为130fs,重复频率为1kHz.首先利用飞秒激光脉冲能量与烧蚀点大小的关系,得到飞秒激光烧蚀PZT厚膜的能量阈值为100J/cm2,然后利用功率为50～300mW的飞秒激光对PZT压电厚膜进行烧蚀和图形化研究.结合飞秒激光加工的无热影响和无掩膜等特性,在一定的扫描控制下,可以实现对PZT厚膜高精度和高效率的加工.最后,在功率为300mW、扫描速率为5mm/s条件下实现了对PZT厚膜驱动器的图形化.经测试表明,图形化以后,PZT驱动器驱动性能有明显(30%)地提高.